Integratable capacitative pressure sensor and process for its manufacture

ABSTRACT

A process for manufacturing an integratable capacitative pressure sensor includes the following steps, starting from a semiconductor substrate (1): application of a guard film (2), precipitation of a polycrystalline semiconductor film (4), doping the polycrystalline semiconductor film (4) and removal of the guard film (2) by etching; then insulating the semiconductor zone (7) from the semiconductor substrate (1), and applying an insulator film (8) on the insulated semiconductor zone (7). The pressure sensor product, which is compatible with CMOS circuits and has increased sensor accuracy, includes a semiconductor zone (7) insulated from the semiconductor substrate (1) and an insulator film (8) applied on the insulated semiconductor zone (7), the polycrystalline semiconductor film (4) being located on the insulator film (8) above the insulated semiconductor zone (7).

DESCRIPTION

The present invention relates to an integratable capacitative pressuresensor, particularly of the type which includes a semiconductorsubstrate and a polycrystalline semiconductor film, which define apressure sensor cavity and which is provided with a dopant at leastwithin a diaphragm-like area located above the pressure sensor cavity; aprocess for manufacturing such an integratable capacitative pressuresensor; and an integratable capacitative pressure sensor array.

A pressure sensor of the type in question as well as a process for itsmanufacture are already known from the following technical publications:

H. Guckel and D. W. Burns, "Planar Processed Polysilicon Sealed Cavitiesfor Pressure Transducer Arrays", IEDM 1984, pages 223 to 225.

H. Guckel and D. W. Burns, "A Technology for Integrated Transducers",Transducers 185, Philadelphia 1985, pages 90 to 92.

Such a prior art pressure sensor is shown in FIG. 6. In the process formanufacturing this known pressure sensor, a silicon substrate 1 hasfirst applied thereto a spacer film 2, which is also referred to asspacer. This spacer film 2 defines a future pressure sensor cavity 3.The spacer film 2 has deposited thereon a polysilicon film 4.

FIG. 7 shows a top view of the pressure sensor according to FIG. 6. Asis clearly evident especially from FIG. 7, the spacer film 2 is providedwith extensions extending through the polysilicon film 4 and definingetching passages 5 which permit the guard film 2 to be removed frombelow the polysilicon film 4 by means of etching. When the spacer film 2has been etched away, the etching passages 5 will be closed. Dependingon the process executed, a vacuum or a gas pressure, which can beadjusted to a defined value, will remain in the pressure sensor cavity3. The polysilicon film 4 has a diaphragm-like structure, which isadapted to be deformed by external pressure. The degree of deformationcan be converted into an electrical signal by applied piezo-resistiveresistors.

The deformation of the diaphragm, which consists of the polysilicon film4, can also be detected capacitively, as is, for example, explained inthe following technical publication:

M. M. Farooqui and A. G. R. Evans, "A Polysilicon-Diaphragm-BasedPressure Sensor Technology", Y. Phys. E. Sci. Inst. 20 (1987), pages1469 to 1471.

For capacitively detecting the deformation of the diaphragm-likepolysilicon film 4, the polysilicon film 4 can be doped heavily byimplantation in the area of a diaphragm region 6, whereby acounterelectrode to the electrode formed by the substrate 1 is produced,as will especially be evident from FIG. 8.

Such a known pressure sensor is, however, not compatible with CMOS(complementary metal oxide semiconductor) circuits. Furthermore, thecapacitance of the known pressure sensor depends on the voltage applied,since a non-ohmic resistance is created between the polysilicon film 4and the silicon substrate 1. Furthermore, due to the resistance formedby the non-implanted region of the polysilicon film 4, the charge of theknown pressure sensor has to be detected with a specific frequency.Hence, the known pressure sensor is not suitable for detecting pressurechanges by means of low-frequency sampling read-out circuits. Nor canthe known pressure sensor be used for monolithic integration withadditional electronic circuit elements.

German Patent DE 37 23 561 Al discloses a capacitative pressure sensorstructure having a multi-layer, comparatively complicated structure. Alower insulating layer is formed on a substrate, the lower insulatinglayer having provided thereon a diaphragm support layer, which enclosesa pressure sensor cavity. The diaphragm support layer is coated with acover layer for closing the pressure sensor cavity. Only the diaphragmsupport layer, but not the cover layer, consists of a polycrystallinesemiconductor material. A semiconductor zone, whose dimensionscorrespond essentially to those of the pressure sensor cavity locatedabove the semiconductor zone, is defined within the substrate by adoping opposite to the doping of the substrate.

In the case of such a pressure sensor, the total capacitance of thesensor element is determined, on the one hand, by a pressure-dependentcapacitance in the area of the diaphragm and, on the other hand, by apressure-independent capacitance, which is essentially determined by thecapacitance of the diaphragm supporting portion with respect to thesubstrate. Due to the small space between the diaphragm supportingportion and the substrate, the pressure-independent capacitance amountsto approximately 95 percent based upon the total capacitance value. Inview of the fact that, in FIG. 1A of DE 37 23 561 Al, the diaphragmsupporting portion is positioned above the non-doped area of thesubstrate, a dependence on the temperature and on the electric voltageof the pressure-independent capacitance component can only be reduced byinsulating the conductive diaphragm portion from the polysilicon supportlayer by means of an upper insulation layer. If, in the case of thispressure sensor structure, the whole pressure sensor area which islocated above the substrate and which determines the pressure sensorcavity were produced from one single material, the high,pressure-independent capacitance component would become excessivelydependent on temperatures and voltages for the above-mentioned reasons.

It is the principal object of the present invention to provide a processfor manufacturing a pressure sensor, as well as to provide a pressuresensor of the type mentioned above, in such a way that improved pressuresensor accuracy is achieved in combination with a simplification of thepressure sensor structure and a simplification of the manufacturingprocess.

In accordance with the present invention, it is possible to produce thepart of the sensor body which encloses the pressure sensor cavity byprecipitating one single polycrystalline semiconductor film, and thisdoes not result in an excessively large temperature-dependent andvoltage-dependent, pressure-independent capacitance component becausethe polycrystalline semiconductor film is located above the insulatorfilm on the insulated semiconductor zone.

It is a further object of the present invention to provide anintegratable pressure sensor array for measuring the pressureaccurately.

In the following detailed description, preferred embodiments of thepressure sensor according to the invention as well as of pressure sensorarrays according to the invention will be explained in detail withreference to the accompanying drawings, in which:

FIG. 1A shows a cross-sectional view of a first embodiment of thepressure sensor according to the present invention;

FIG. 1B shows a cross-sectional view of a second embodiment of thepressure sensor according to the present invention;

FIG. 1C shows a cross-sectional view of a third embodiment of thepressure sensor according to the present invention;

FIG. 2 shows a top view of an embodiment of a pressure sensor arrayaccording to the present invention;

FIG. 3 shows a sectional view of an additional embodiment of a pressuresensor array according to the present invention;

FIG. 4 and 5 show embodiments of the pressure sensor array according tothe present invention in the form of circuits;

FIG. 6 shows a cross-sectional view through a known pressure sensor;

FIG. 7 shows a top view of the known pressure sensor shown in FIG. 1;and

FIG. 8 shows a cross-sectional view of an additional, known pressuresensor.

In FIG. 1 to 5, reference numerals, which correspond to the referencenumerals used in FIG. 6 to 8, refer to identical or to similar parts sothat a renewed explanation of these parts can be dispensed with.

The process for producing the embodiment of the pressure sensor shown inFIG. 1A differs from the initially described manufacturing processaccording to the prior art with regard to the fact that, prior to theprocess step of applying the spacer film 2, the silicon substrate 1 isprovided with a doping, which is chosen to be opposite to theconductivity type of the substrate, within a doping zone 7. It followsthat, in the case of the p-substrate 1 shown, an n⁺ -doped area 7 isgenerated, on the one hand for the purpose of producing a highlyconductive electrode and, on the other hand, for the purpose ofinsulating this electrode, which is formed by the doped area 7, from thesilicon substrate 1 by the pn-junction. Subsequently, an insulator film8, which may consist e.g. of Si₃ N₄, is applied to the doped area 7 thusformed. The spacer film 2 and the polysilicon film 4 are applied to theinsulator film 8 in the manner described at the beginning, whereupon theguard film is removed by etching. The etching passages described at thebeginning are then closed.

As can be seen in FIG. 1B, the step of producing a pn-junction forinsulating the area 7 from the substrate 1 can also be replaced by thestep of producing a buried insulation film 9, which will insulate thearea 7, in the semiconductor substrate 1 by implantation of suitableimplantation substances. The buried insulation layer 9 can consist ofSiO₂ or of Si₃ N₄. When the area 7 is insulated from the substrate 1 inthis way, the area 7 will be annealed thermally after oxygenimplantation.

FIG. 1C shows a third embodiment of the invented pressure sensor, whichdiffers from the embodiment according to FIG. 1A with regard to the factthat the doped area 7 is subdivided into a first doped area 7A, whichforms an electrode of the pressure sensor capacitance, and in a seconddoped area 7B, which is arranged only below the supporting area of thepolysilicon film 4. This structuring of the doped area 7 has the effectthat the capacitance of the support no longer lies in the signal path sothat the influences of this pressure-independent capacitance of thesupport can be reduced substantially.

In the invented pressure sensor, it is expedient to choose the doping ofthe doped area 7, 7B below the supporting area of the polycrystallinesemiconductor film 4 so high that a metallic behavior in the range ofdegeneration will result therefrom; this reduces the influences of thetemperature-dependent capacitance variation as well as of thevoltage-dependent capacitance variation of the pressure-independentcapacitance component of the sensor still further.

In any case, the invented pressure sensor structure constitutes apotential-free capacitor which is compatible with CMOS circuit elementsand which permits integration within a CMOS circuit. The capacitance ofthe pressure sensor according to the invention depends only on thepressure, but not on the voltage applied. Due to the fact that thepolysilicon film 4 is fully insulated from the silicon substrate 1, thecapacitance of the pressure sensor can be detected in a quasi-staticstate as well as with high a high read-out frequency.

In the pressure sensor according to the present invention, the wholepolysilicon film 4 can be provided with a dopant so as to increase itsconductivity. A delimitation of the doped area, which is necessary inthe case of the prior art, can be, but need not be effected in the caseof the pressure sensor according to the present invention.

The useful capacitance of a single pressure sensor shown in FIG. 1A, 1B,1C amounts, typically, to a few femtofarad. In the invented pressuresensor structure, it is, however, possible to arrange a plurality ofsuch pressure sensors on a silicon substrate 1 in a field-like array.

As can be seen from FIG. 2, such pressure sensors D, which are arrangedin a field-like array, can be interconnected by connection arms 9 of thepolysilicon film 4 and can thus be connected in parallel for increasingthe capacitance so as to obtain a higher output signal.

Depending on the type of interconnection used, it is also possible todetect the capacitance value of each individual pressure sensor D in thecase of such a field-like array separately so as to measure pressuredistributions in relation to the respective location or so as toconstruct a redundant pressure sensor system. Defective pressure sensorscan then be detected so that these pressure sensors will no longercontribute to the total signal.

It is also possible that, in the case of such a field-like array, thediameters and/or the diaphragm thicknesses may vary from one pressuresensor to the next on one chip so that various pressure ranges can bedetected with the sensors of a single chip.

The invented pressure sensor has a pressure-independent as well as apressure-dependent capacitance component. The pressure-independentcapacitance component consists e.g. of conductor capacitances as well asof the capacitance between the base of the polysilicon film 4 and thesilicon substrate 1. Furthermore, the invented pressure sensor may besubjected to pressure-independent capacitance variations caused by theinfluence of temperature. The above-mentioned pressure-independentcapacitance components or capacitance variations can be compensated inthe case of a pressure sensor array, which is based on the inventedpressure sensor and which includes a pressure sensor and a referenceelement.

FIG. 3 shows such an integratable, capacitative pressure sensor arrayincluding, on the left-hand side, a pressure sensor having the structuredescribed hereinbefore and, on the right-hand side, a capacitativereference element. The only difference between the reference elementaccording to the present invention and the pressure sensor according tothe present invention is that the reference element has a polysiliconfilm 4 of increased flexural strength. In the embodiment shown, thisincreased flexural strength can be obtained by applying an additionalfilm 10 to the polysilicon film 4 of the reference element 4. This can,for example, be done by precipitation of polysilicon. It is alsopossible to provide the reference element with a polysilicon film 4whose thickness exceeds that of the pressure sensor.

A pressure sensor array including a pressure sensor and a referenceelement of the type shown in FIG. 3, permits a measurement of pressureby detecting the capacitance difference between the capacitance of thepressure sensor and that of the reference element. This has the effectthat all capacitances and capacitance variations which are not pressuredependent will be compensated. In view of the fact that the pressuresensor and the reference element can be constructed on the samesubstrate 1 by the same manufacturing steps, the best possiblesimilarity between the structure of the pressure sensor and that of theassociated reference element will be obtained.

The measurement of pressures by means of a pressure sensor/referenceelement pair can preferably be carried out in an expedient manner by socalled capacitance measuring circuits of the switched capacitor type,which are shown in FIG. 4 and 5.

The capacitance measuring circuit of the switched capacitor type shownin FIG. 4 is provided with reference numeral 11 in its entirety andcomprises an operational amplifier 12 whose non-inverting input isconnected to ground. In a first control state a, the pressure sensorC_(sens) has its first electrode connected to a first potential V₁ via afirst switch S₁ . and, in a second control state b, it has its firstelectrode connected to a second potential V₂ via a second switch S₂. Thereference element C_(ref) is positioned between the inverting input andthe output of the operational amplifier, and a third switch S₃, which isclosed in the first control state a and open in the second control stateb, is connected in parallel with the reference element.

To the person skilled in the art, it will be evident that the outputvoltage V_(out) of the capacitance measuring circuit of the switchedcapacitor type is proportional to the quotient of the capacitance of thepressure sensor C_(sens) to that of the reference element C_(ref).

Deviating from the embodiment of the capacitance measuring circuit ofthe switched capacitor type shown in FIG. 4, the pressure sensorC_(sens) shown in FIG. 4 and the reference element C_(ref) shown in thesame figure can be changed round. Accordingly, a reciprocal dependenceof the output voltage V_(out) of circuit 11 will be obtained.

FIG. 5 shows a different embodiment of the capacitance measuring circuitof the switched capacitor type, which is here provided with referencenumeral 13 in its entirety. Circuit components and control statescorresponding to the embodiment of FIG. 4 are provided with the samereference numeral. Hence, the description following below can berestricted to an explanation of the features of circuit 13 which deviatefrom circuit 11 according to FIG. 4. In this embodiment, a secondreference element C_(ref2) has its second electrode connected to theinverting input of the operational amplifier 12. In a first controlstate a, the second reference element C_(ref2) has applied to its firstelectrode a fourth potential V₄ via a fifth switch S₅, whereas, in asecond control state b, it has applied thereto a third potential V₃ viaa fourth switch S₄. The output voltage V_(out) of this capacitancemeasuring circuit of the switched capacitor type 13 is proportional tothe quotient of the capacitance of the pressure sensor C_(sens) to thatof the first reference element C_(ref1) minus the quotient of thecapacitance of the second reference element C_(ref2) to that of thefirst reference element C_(ref1). In such a circuit, additivecapacitance terms will drop out so that the output voltage will dependonly on the pressure to be measured.

What is claimed is:
 1. A process for manufacturing an integratablecapacitative pressure sensor comprising the following steps, startingfrom a semiconductor substrate:insulating a semiconductor zone (7) fromthe semiconductor substrate (1); applying an insulator film (8) on theinsulated semiconductor zone (7); producing a spacer film (2), whichdetermines a future pressure sensor cavity (3) and which is locatedabove the semiconductor zone (7) of the semiconductor substrate (1);precipitating a polycrystalline semiconductor film (4) in such a waythat this polycrystalline semiconductor film covers the spacer film (2)at least partially and that the polycrystalline semiconductor film (4)is located on the insulator film (8) above the insulated semiconductorzone (7) such that the polycrystalline semiconductor film (4) does notextend beyond the semiconductor zone (7) in the lateral directionthereof; doping the precipitated polycrystalline semiconductor film (4)at least within a diaphragm-like area located above the spacer film (2);and removing the spacer film (2) by etching.
 2. A process according toclaim 1, wherein the step of insulating the semiconductor zone (7) fromthe semiconductor substrate (1) includes the production of a pn-junctionin said semiconductor substrate (1) by adequate doping of saidsemiconductor zone (7).
 3. A process according to claim 1 wherein thestep of insulating the semiconductor zone (7) includes the implantationof implantation substances suitable for insulation so as to produce aburied insulation film in said semiconductor substrate (1).
 4. A processaccording to claim 3, wherein the implantation substances are selectedfrom the group consisting of oxygen and nitrogen.
 5. A process accordingto claim 3, further comprising the step of thermally annealing thesemiconductor zone (7) defined by the buried insulation film, said stepbeing carried out after the implantation step.
 6. A process according toclaim 1, wherein the insulator film (8) includes Si₃ N₄.
 7. A processaccording to claim 1, wherein the spacer film (2) is constructed suchthat it extends through the polycrystalline semiconductor film (4),thereby defining at least one etching passage (5).